U.S. patent application Ser. No. 12/214,345, filed Jun. 18, 2008, which is still pending and incorporated herein by reference, describes a method for producing armor through metallic encapsulation of a ceramic core. In summary, this method involves first selecting a ceramic tile of the desired geometry; fabricating a conformal sheet metal container with dimensions modestly oversized relative to the ceramic tile; and placing the ceramic tile in the conformal sheet metal container and closing it. The closed conformal sheet metal container and a bed of granular material, which serves as a pressure transmission vehicle, are placed in an isostatic pressurization chamber. After the isostatic pressurization container is closed, degassed and hermetically sealed, it is subjected to temperature and pressure cycles that cause diffusion bonding of the ceramic tile and the conformal sheet metal container.
The method described in U.S. patent application Ser. No. 12/214,345 has many advantages over the prior art. It is less complicated, less costly, capable of working with a wide range of metal and ceramic materials combinations, and also compatible with the requirements of reproducible and large-scale manufacturing. However, the method requires a hot isostatic pressurizing step, or high pressure step, which is complex and expensive.
It is an object of the present invention to use lower pressure methods for bonding metals, such as titanium, aluminum and magnesium, to ceramics, such as silicon carbide, boron carbide, titanium diboride, and alumina. Lower pressure processing methods will allow the use of lower cost, much more widely available, autoclaves or brazing furnaces.
The use of these lower pressure bonding methods is based on the use of a “wetting” braze with a relatively low temperature melting point (herein defined as an “interlayer material”), as exemplified by aluminum 4047 alloy. Because it appears that an interlayer material, such as aluminum 4047 alloy, produces effective wetting of both the metal and ceramic, the relatively low melting point interlayer material produces an effective bonding. In other words, it is not necessary to depend exclusively on diffusion bonding, as it is classically carried out in a hot isostatic press, which entails use of relatively high temperatures and high pressures, to initiate plastic flow and chemical diffusional bonding at interface. In effect, a properly designed interlayer material with the right technical properties can achieve many of the same attributes that result from high pressure diffusion bonding.
The present invention is a method for joining titanium, aluminum or magnesium sheet to armor ceramics using an autoclave furnace, which works at a few hundred psi, or even a conventional brazing furnace, which works at atmospheric or slightly positive pressure. The key for making the low pressure bonding work is to have an interlayer material and a soft, ductile metallic foil “bag” (herein defined as a “metallic bag”) that can be vacuum sealed. The parts to be bonded, here a ceramic tile, an interlayer material and conformal sheet metal container, are placed in a metallic bag, the metallic bag is sealed and evacuated, and the metallic bag is placed into an autoclave or brazing furnace.